Method and Apparatus For Selecting Between Available Neighbors in a RAPID Alternate Path Calculation

ABSTRACT

A weighting process may be used to select between alternate neighbors in a RAPID calculation to enable policy and/or traffic engineering considerations to affect the selection of an alternate path through the network. The information used to weight the neighbors may static administratively assigned weighting information or dynamic weighting information such as local statistical traffic condition information. The process may take into account the amount of traffic being handled by the current primary next hop for the destination, the available capacity of the available alternate neighbors, the ability of the alternate neighbors to handle the additional traffic, and other considerations. Weighting may occur after a set of available loop free alternate neighbors has been determined. Alternatively, weighting may occur before the RAPID calculation has been performed to cause the neighbors to be ordered prior to RAPID processing. This may enable RAPID calculation to stop without considering all available neighbors.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/512,671, filed Aug. 30, 2006, and is related to co-pending U.S.patent application Ser. No. 11/410,747, entitled Method And ApparatusFor Simplifying The Computation Of Alternate Network Paths, filed onApr. 25, 2006, the content of each of which is hereby incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication networks and, moreparticularly, to a method and apparatus for selecting between availableneighbors in a RAPID alternate path calculation.

2. Description of the Related Art

Data communication networks may include various computers, servers,nodes, routers, switches, bridges, hubs, proxies, and other networkdevices coupled to and configured to pass data to one another. Thesedevices will be referred to herein as “network elements.” Data iscommunicated through the data communication network by passing protocoldata units, such as Internet Protocol packets, Ethernet Frames, datacells, segments, or other logical associations of bits/bytes of data,between the network elements by utilizing one or more communicationlinks between the network elements. A particular protocol data unit maybe handled by multiple network elements and cross multiple communicationlinks as it travels between its source and its destination over thenetwork.

The various network elements on the communication network communicatewith each other using predefined sets of rules, referred to herein asprotocols. Different protocols are used to govern different aspects ofthe communication, such as how signals should be formed for transmissionbetween network elements, various aspects of what the protocol dataunits should look like, how protocol data units should be handled orrouted through the network by the network elements, and how informationsuch as routing information should be exchanged between the networkelements. Protocol Data Units will also be referred to herein aspackets.

There are several different types of network routing protocols, oneclass of which is commonly referred to as link state routing protocols.Link state routing protocols assign cost metrics to each link on thenetwork, and the routers advertise the links and costs through the useof link state advertisements. The routers collect the link stateadvertisements and build a link state database containing informationassociated with links on the network. This network view enables therouters to compute lowest cost paths through the network to intendeddestinations. These calculations are performed in advance and thenprogrammed into the data plane of the network element. In operation,when a packet arrives, the data plane will automatically forward thepacket over the lowest cost path toward its intended destination.Several examples of routing protocols that operate in this mannerinclude Intermediate System to Intermediate System (IS-IS) and OpenShortest Path First (OSPF), although other link state routing protocolsexist and may be developed as well.

Network failures, such as link failures and node failures, may occur ina communication network. When a failure occurs, traffic that is intendedto flow through the failure must be routed around the failure so that itis able to traverse the network. Many different ways of handling networkfailures have been devised over the years. For particular types oftraffic, it is generally considered desirable or necessary to enabletraffic to be switched to an alternate path with 50 milliseconds offailure on the primary path, so that real time traffic being carried bythe network is not affected by the failure. While this is generallypossible using physical layer protection switching, such as by switchingtraffic to a protection path using SONET equipment, it would beadvantageous to be able to provide this type of protection at therouting layer.

When a failure occurs in a network implementing a link state routingprotocol, the local router will react to the failure by generating andflooding new routing updates to other routers in the network, perhapsafter a hold-down delay. Upon receipt of the routing update, all therouters in the network will re-compute routes through the network basedon the new network topology. These routers will then load the revisedforwarding tables into the forwarding hardware. The convergence time forthis process to complete may be on the order of several seconds.Accordingly, use of the normal link state routing protocol pathcomputation mechanisms to provide sub-50 ms failure recovery isgenerally not tenable.

One way to provide fast failover to an alternate network path is throughthe use of pre-computed alternate paths that are installed in the dataplane along with the primary path. For example, when a router initiallycomputes a path to a destination, the router may also assume a failureon its primary path and compute an alternate path to the destination atthe same time. The alternate path may then be programmed into the dataplane of the network element so that, if a failure on the primary pathoccurs, the alternate path may be used to forward traffic temporarilywhile new primary paths are being computed.

FIG. 1 shows a simplified example of a network 10 including six nodes 12interconnected by links 14. The cost of the links in this figure will beassumed to be symmetric, and are shown as numbers on the links In anactual implementation, the network may be much larger and include alarger number of nodes. In this example, traffic is flowing from R1 toR6. Initially, a shortest path 20 from R1 to R6 will be through nodes R2and R3, since this path has a cost of 4.

FIG. 2 shows the network of FIG. 1 in which there has been a failure onthe link between R2 and R3. Although R2 will sense the failure, R1 willnot know of the failure and will continue to send traffic to R2 to beforwarded to R6. To enable R2 to continue to forward traffic to R6, R2will have pre-computed an alternate path through the network and haveprogrammed that alternate path into its data plane. For example, in theexample shown in FIG. 2, the pre-computed alternate path may be totransmit the data to R4, which may then transmit the data to thedestination (R6) over its own shortest path.

The failure on link 2 will eventually be advertised by R2 using astandard Link State Advertisement (LSA), so that each of the nodes onthe network may recompute paths through the network using the updatednetwork information. These new paths will then be used by the networkelements in a standard manner. For example, since the path from R1 to R6via R4 and R5 has a cost of 6, R1 will stop sending R2 traffic intendedfor R6 once the new paths are computed and installed by the networkelements. Having pre-computed alternate paths, however, enables thenetwork elements to continue forwarding traffic to intended destinationswhile the network nodes recompute new primary paths through the newnetwork topography.

To determine which paths are able to be used to forward traffic in thismanner, each router must determine which neighbors are loop free. In thenetworks shown in FIGS. 1 and 2, R4 is loop free because the distancefrom R4 to R6 is less than the distance from R4 to R2 plus the distancefrom R2 to R6. Stated another way, when R2 sends traffic to R4 to beforwarded to R6, R2 needs to know that the shortest path from R4 to R6does not require R4 to forward traffic back through R2. Since R4 willnot know of the failure on the link from R2 to R3, having R4 return thetraffic to R2 would cause a routing loop to occur, and would defeat thepurpose of trying to send out the traffic to R6 via an alternate paththrough the network. Accordingly, to enable alternate path routing to beenabled, each router must determine which of its neighbors has a lowestcost path to a particular destination that does not cause traffic to beforwarded back through it, i.e., the nodes must determine whichneighboring routers are on loop free paths to the intended destination.The well-known Dijkstra algorithm may be used to perform thiscalculation.

FIG. 3 shows an example where it will be assumed that R2 does not haveany loop-free neighbors. In this case, R2 may be able to use router R1as its alternate path if router R1 is configured to enable U-turns tooccur in the event of a failure on the network. Enabling U-turns of thisnature is also described in greater detail in U.S. Patent ApplicationPublication No. US2005/0073958A1, the content of which is herebyincorporated herein by reference. Essentially, U-turns enable the numberof source/destination pairs that are protected on the network to beincreased. To enable U-turns, R1 must be capable of breaking U-turns,and must have a loop free node-protecting alternate path to reach thedestination.

Once the node has determined which neighbors provide loop-free alternatepaths, the node must select one of the neighbors to be used for thealternate path. Conventionally, this has been done by looking forneighbors with paths to the destination that avoid the immediatedown-stream node, such as node R3 in the illustrated example. Wherethere is more than one such neighbor, or where no neighbor's path avoidsthe immediate downstream node, the selection of a neighbor from equallypreferable available neighbors has been random or based on the linkweights used for the shortest path calculation. While this isstraightforward and easy to implement, it may be advantageous to providean alternate mechanism for discerning between available neighbors in aRAPID calculation.

SUMMARY OF THE INVENTION

A weighting process may be used to select between alternate neighbors ina RAPID calculation to enable policy and/or traffic engineeringconsiderations to affect the selection of an alternate path through thenetwork. The information used to weight the neighbors may static, forexample network administrator assigned preference information, ordynamic such as local traffic statistic information. The process maytake into account the amount of traffic being handled by the currentprimary next hop for the destination, the available capacity of theavailable alternate neighbors, the ability of the alternate neighbors tohandle the additional traffic, and other similar considerations that areable to be derived from the local traffic statistic information. Theprocess may select one neighbor or a group of neighbors to serve asalternates for a given destination. Weighting may occur after a set ofavailable loop free alternate neighbors has been determined.Alternatively, weighting may occur before the RAPID calculation has beenperformed to cause the neighbors to be ordered prior to RAPIDprocessing. In this embodiment the first neighbor that is found to be aloop free alternate or U-turn loop free alternate may be selected as thealternate path to the destination and processing of the remainingneighbors may be stopped to reduce the computational complexity of theRAPID process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present invention are pointed out with particularity inthe appended claims. The present invention is illustrated by way ofexample in the following drawings in which like references indicatesimilar elements. The following drawings disclose various embodiments ofthe present invention for purposes of illustration only and are notintended to limit the scope of the invention. For purposes of clarity,not every component may be labeled in every figure. In the figures:

FIG. 1 is a functional block diagram of a portion of an examplecommunication network showing an initial path through the network;

FIG. 2 is a functional block diagram of the network of FIG. 1 showing analternate path through the network via a neighboring node;

FIG. 3 is a functional block diagram of the network of FIG. 1 showing analternate path through the network that relies on a U-turn;

FIG. 4 is a functional block diagram of a network in which a sendingnode has a large number of neighboring nodes;

FIG. 5 is a functional block diagram of a network element according toan embodiment of the invention;

FIG. 6 is a flow chart illustrating a process of selecting an alternatenetwork path according to an embodiment of the invention; and

FIG. 7 is a flow chart illustrating a process of selecting an alternatenetwork path according to another embodiment of the invention.

DETAILED DESCRIPTION

The following detailed description sets forth numerous specific detailsto provide a thorough understanding of the invention. However, thoseskilled in the art will appreciate that the invention may be practicedwithout these specific details. In other instances, well-known methods,procedures, components, protocols, algorithms, and circuits have notbeen described in detail so as not to obscure the invention.

Reliable Alternate Paths for IP Destinations (RAPID) sets forth aprocess whereby, for each destination, a node will calculate which ofits neighbors may be used to forward traffic to the destination uponoccurrence of a failure in the network. According to an embodiment ofthe invention, once a set of alternate neighbors has been determinedusing this process, selection between those alternate neighbors may lookat operator specified policy weightings or dynamic network usageinformation to select from the available alternate neighbors. Thepreferential selection process may take place on all available neighborsor may take place first on loop free neighbors and then on U-turnneighbors. Alternately, the neighbors may be ranked first and thenchecked, in order, to determine which of the neighbors is loop free andmost preferable.

FIG. 4 illustrates a portion of an example of a network in which asource node S has a large number of neighbors N1-Ni that may potentiallybe able to forward traffic to destination nodes D1-Dj upon failure ofprimary next hop E. To determine which neighbors may be used to forwardtraffic to a particular destination, the source node S will determinewhich of the neighbors are able to provide a loop free alternate path tothe destination. Determining which neighbors provide loop free alternatepaths is well known in the art, as described above. In the examplenetwork illustrated in FIG. 4, neighbors N2 and N3 are capable ofserving as alternates to destination D1, neighbor N3 is capable ofserving as an alternate to destination D2, neighbors N2, N3, N4, and N5are capable of serving as alternates to destination D3, and neighbor N5is capable of serving as an alternate to destination Dj. Additionally,one or more of neighbors N1-Ni may be a U-turn neighbor.

Focusing on destination D3, in this example there are four neighborsthat may serve as alternate next hops should primary next hop E fail.According to an embodiment of the invention, the node S may select oneor more of these neighbors to be used as alternates from this set ofloop free neighbors by using static or dynamic policy weightinginformation. Additional information associated with the manner in whichone of the neighbors is selected is presented below.

FIG. 5 shows an example of a network element 12 that may be configuredto implement an embodiment of the invention. As shown in FIG. 5, thenetwork element 12 includes a control plane 40 and a data plane 60. Thecontrol plane 40 is generally configured to instruct the data plane 60how to handle data on the network. The data plane 60 is generallyconfigured to handle high speed data traffic, such as packet traffic onan IP network. For example, the data plane may include one or more I/Ocards 62 interconnected by a switch fabric 64. Routing tables, includingalternate paths determined via the alternate path calculation processdescribed herein, may be programmed into the data plane to enable thedata plane to handle data on the network. Many data plane architecturesmay be used in connection with the network element of FIG. 5, and theinvention is not limited to a particular data plane architectureselected to implement an embodiment of the invention.

The control plane 40, in this embodiment, includes a processor 42containing control logic 44 that is able to be programmed to enable thenetwork element to perform the functions described herein to computealternate paths through the network. For example, the network elementmay contain a memory 46 containing software such as alternate pathsoftware 48 and routing software 50 configured to enable the networkelement to select primary and alternate paths to destinations on thenetwork.

The memory may also contain one or more tables, such as link statedatabase 52 that contains the data to be used by the routing software 50and/or alternate path software 48 to enable the network element tocalculate which neighbors may provide loop free alternate paths to thedestinations. The memory may also contain a copy of the current routingtables 54 that have been programmed into the data plane, and otherinformation commonly maintained by the network element to enable it tofunction on the network. To enable the network element 12 to selectbetween available neighbors that can provide loop free alternate paths,the network element may also have a weighting database 56 containingstatic policy information and/or a dynamic weighting information. Itshould be understood that the invention is not limited to a networkelement configured in the manner discussed above, as numerous otherarchitectures may be used to create a network element.

According to an embodiment of the invention, once a set of potentialloop free alternate neighbors has been calculated, one or more of theloop free alternate neighbors is selected by applying a set of static ordynamic weights. FIGS. 6 and 7 illustrate two of possible ways in whichthis may be implemented by the alternate path software 48 according toembodiments of the invention.

In the embodiment shown in FIG. 6, for each destination D (100), thealternate path software will perform a RAPID calculation to determine aset of potential loop free alternate neighbors that may be used as nexthop alternates should there be a failure on the primary path throughprimary neighbor E (102). If no neighbor may provide a loop freealternate path, the alternate path software may determine whether thereare any neighbors that can provide a U-turn alternate path. Once the setof possible next hop alternates (or U-turn alternates) has been found,weighting information will be applied to select one of the neighborsfrom the set. Optionally, available loop free neighbors may be furtherdivided into two groups according to whether they provide a nodeprotecting loop free path to the destination or a link protecting loopfree path to the destination, before applying weighting information toselect between the available loop free or U-turn neighbors.

The step of applying weighting information to select one of theneighbors from the set may be performed in a number of different ways,depending on the type of weighting information to be applied. Forexample, the alternate path software may apply static weightinginformation such as administratively assigned policy link weights (110)that have been assigned by a network manager (112). Static weights mayenable pre-determined secondary policy weights to be used as linkweights so that the nodes on the network may determine the path to thedestination that uses the highest valued alternate path. For example,assume in FIG. 4 that the network manager had assigned the values setforth in Table I to the links from node S to each of the neighboringnodes N1-Ni. The assigned policy values are also shown on the links inFIG. 4 for convenience.

TABLE I Link Value S → N1 3 S → N2 3 S → N3 2 S → N4 5 S → N5 1 S → N6 4S → Ni 5

In the first part of the process, the alternate path software willdetermine which of the nodes N1-Ni is a loop free alternate. In theexample shown in FIG. 4, with respect to destination D3, nodes N2, N3,N4, and N5 are all potential loop free alternates. Hence, the alternatepath software 48 will form a set N containing nodes N2, N3, N4, and N5.

The alternate path software will then look into the static policy tableand determine that Node N2 has a value of 3, Node N3 has a value of 2,Node N4 has a value of 5, and Node N5 has a value of 1. Assuming thatthe alternate path software is looking for the node that has the highestpolicy value (114), the alternate path software will choose node N4 asthe alternate neighbor for destination D3. If the alternate pathsoftware was configured to look for the node with the lowest policyvalue, the alternate path software would have chosen node N5 since thatnode has the lowest policy value.

A situation may arise where the weighting process of selecting betweenneighbors using the static weighting information (or the dynamicweighting information discussed below) results in a determination thattwo or more neighbors are equally preferable. In this situation, amechanism similar to Equal Cost Multipath Protocol (ECMP) may be used toselect one of the neighbors as an alternate. Other mechanisms ofbreaking a tie may be used as well and the invention is not limited tothe use of a mechanism similar to ECMP.

Once the alternate has been selected, the process will end with respectto this destination and the alternate path will be programmed into thedata plane (106) in a conventional manner so that the network elementmay use the alternate path upon occurrence of a failure on the network.

Optionally, to enable alternate paths to be distributed across multipleneighbors, the policy values may change periodically as new destinationsare considered by the alternate path software. For example, it may notbe desirable from a traffic engineering standpoint to cause all trafficfrom a given node to pass through a single alternate neighbor. If thepolicy information is such that a particular neighbor has a high (orlow) policy value, that neighbor will be preferentially selected overother neighbors whenever the neighbor is able to provide a loop freealternate path to a particular destination. Accordingly, when a neighboris selected to provide a loop free alternate path, the policy valueassociated with that neighbor may be reduced (or increased) by aparticular value for a number of destination calculations so that otherneighbors are more likely to be selected as alternates for some of theother destinations.

For example, assume as set forth in Table I above, that node N4 has beenselected as an alternate for destination D3. The value associated withnode N4 may be reduced by 3 during the subsequent calculation ofalternates for destination D4 so that, if node N4 is part of the set Nfor destination D4, it is less likely to be selected as an alternate.The reduction in policy value may decay over time to return the policyvalue associated with node N4 to its original value if it has notrecently been selected as an alternate. Other ways of adjusting thepolicy values may be used as well to help distribute the traffic on thenetwork and the invention is not limited to this particularimplementation.

Although the weighting table in this embodiment shows only one level ofpolicy information, the invention is not limited in this manner aspolicy values on other links between the source and the destination maybe considered as well. Thus, the alternate path software may beconfigured to sum the policy values on the links between the source anddestination rather than simply looking at the policy value on the linkbetween the source and the loop free neighbor.

The weighting process may use dynamic weighting information as well asor instead of static weighting information (120). Dynamic weightinginformation may be information relating to network conditions, such aslocal traffic statistic information. The local traffic statisticinformation may include information as to how much traffic is beinghandled by the node S for the destination D, the capacity of theneighbors, the amount of traffic being handled by the neighbors, thetotal or percentage residual capacity on the neighbors, or otherinformation relating to traffic patterns on the network. Optionally,traffic engineering information may be used when available.

As shown in FIG. 6, there are several ways to use dynamic weightinginformation to select a neighbor from available neighbors on thenetwork. For example, dynamic weighting information may be used toselect a neighbor that would be likely to be least impacted by beingrequired to handle traffic from the source to the destinations (122).

To choose the neighbor that would be least likely to be impacted byhandling the traffic, the alternate path software may determine theamount of traffic being handled by the source node S for the destinationD, and look for neighbors with residual capacity that approximates thatamount of traffic. Alternatively, the alternate path software may lookfor neighbors with the largest amount of residual capacity and assignthe neighbor with the largest residual capacity as the alternate forthat destination. The largest residual capacity may be measured in termsof absolute capacity, percentage capacity, or in another way.

For example, if the current primary next hop E is handling 100 Mbps ofdata to destination D3, and only node N3 was capable of handling anadditional 100 Mbps of data, then node N3 would be preferentiallyselected over the other nodes that are capable of providing a loop freealternate path to destination D3. As capacity on neighboring nodes isallocated, the dynamic weighting information database 56 may be updatedto reflect these assignments so that available capacity on neighboringnodes is not oversubscribed.

Selection of neighbors may take place as each destination is consideredor, alternatively, may take place after all destinations have beenconsidered. For example, the alternate path software may compute whichneighbors are able to provide loop free alternate paths to each of thedestinations and then assign neighbors to each destination once thecalculations have completed. Assigning neighbors in this manner mayenable available capacity on neighbors to be better allocated,particularly where there are only one or a few neighbors that are ableto reach some of the destinations. For example, referring to FIG. 4,node N5 is able to provide a loop free alternate path to destinations D3and Dj, whereas the other neighbors in this example are not able toreach node Dj.

To enable a full set of loop free alternate paths to be selected, it maybe desirable to assign neighbor N5 as the alternate for traffic todestination Dj and then adjust the dynamic weighting informationdatabase to reflect that assignment before selecting an alternate fortraffic intended to destination D3. In this manner, if node N5 has onlysufficient capacity to provide an alternate path to either D3 or Dj, andother nodes such as nodes N2, N3, or N4 are able to provide an alternatepath to destination D3, those other nodes may be able to be assigned sothat a complete set of alternate paths may be found.

Although particular examples of ways in which static weightinginformation and dynamic weighting information have been provided to helpexplain operation of embodiments of the invention, the invention is notlimited to these particular embodiments as other embodiments may useother types of static, dynamic, or both static and dynamic weightinginformation.

Similarly, although the foregoing description has described a processthat may be used to select a single alternate neighbor, the invention isnot limited in this manner as groups of neighbors may be selected toserve as an alternate using the weighting process described herein, andthe traffic to the destination may be split between the severalalternates in the group. Thus, the invention is not limited to anembodiment where weighting information is used to select only oneneighbor from a group of possible alternate neighbors but rather extendsto an embodiment in which the weighting information is used to selecttwo or more neighbors that will collectively provide an alternate loopfree path to a given destination. For example, where a subset ofneighbors has sufficient residual capacity to handle traffic to aparticular destination, those in the subset may be selected to providethe alternate path to the destination.

Once the neighbor or neighbors has been selected, the selection will beprogrammed into the data plane (126). Programming of a single alternateinto the data plane may be performed in any desired manner, depending onthe configuration of the data plane, and the invention is not limited toany particular manner in which the data plane is programmed. Similarly,multiple alternates may be programmed into the data plane using knowntechniques, such as by using Bloom filters, and the invention is thusnot limited to any particular way in which multiple alternates areprogrammed into the data plane.

In the description provided above with respect to the step of performingthe RAPID calculation (102), the alternate path software may perform aRAPID calculation on each of the available nodes or, alternatively, maypreferentially perform the RAPID calculation on fewer than all nodes asdiscussed in greater detail in copending U.S. patent application Ser.No. 11/410,747 the content of which is hereby incorporated herein byreference.

In the embodiment shown in FIG. 6, it was assumed that the weightingprocess would be performed after a set of loop free alternate neighborshad been determined. The invention is not limited in this manner,however, as the weighting process may also be performed before a set ofloop free alternate neighbors has been determined, to preferentiallyrank the neighbors for processing.

FIG. 7 shows an example of alternate embodiment in which the weightingprocess is performed first and then the alternate path software performsa Dijkstra calculation to determine whether the neighbor is a loop freealternate path or U-turn alternate path to the destination. Using theweighting process to order the neighbors may enable the calculation ofalternate network paths to terminate once the node finds a single loopfree alternate to the destination to thereby reduce the amount ofcalculation required to find a set of alternate paths to the node'sdestinations. If a loop free alternate is not able to be found, theprocess optionally may be used to rank the available U-turn neighbors,and determine if any of the U-turn neighbors are able to provide analternate path to the destination.

As shown in FIG. 7, for each destination (200) the alternate pathsoftware will apply static or dynamic weighting information to rank theneighbors (202). The ranking process, in this step, will assume that allneighbors are loop free, and will rank the neighbors using staticweighting information, dynamic weighting information, or both, asdiscussed in greater detail above in connection with FIG. 6. In anembodiment where the ranking process ranks the nodes the same for alldestinations, the ranking process may be omitted for subsequentdestinations once it has been performed the first time.

Once the neighbors have been ranked, the alternate path software willselect the highest ranked neighbor in the list (204) and perform theRAPID calculation on the neighbor to determine if it will provide a loopfree alternate path to the destination (206). As a result of the RAPIDcalculation, the alternate path software will be able to determinewhether the highest ranked neighbor provides a loop free alternate(208). If the highest ranked neighbor does not provide a loop freealternate to the destination, the alternate path software will check tosee if the neighbor that was just processed is the last neighbor in thelist (210). If there are additional neighbors in the list, the alternatepath software will return and process the next neighbor in the list. Thealternate path software will iterate this process until a loop freealternate has been found (210) or until the last neighbor has beenconsidered (212). If a loop free alternate is found, calculation will bestopped and that neighbor will be used for an alternate path to thedestination (210).

If no loop free alternate is found, then similar processing may beperformed to find the most preferred U-turn alternate. For example, asshown in FIG. 7, the alternate path software in this embodiment willapply static or dynamic weighting information to rank the U-turnneighbors (214). The alternate path software will then select a U-turnneighbor from the list (216) and perform a RAPID calculation on theselected U-turn neighbor (218). Processing of U-turn neighbors is knownin the art and a detailed description of the particular processing thatis required to determine whether a neighbor may serve as a U-turnalternate to a particular destination has thus been omitted.

If the considered U-turn neighbor is able to break a U-turn and providea loop-free path to the destination (220) the calculation will stop andthat U-turn neighbor will be used as an alternate path for thatdestination. If not, the alternate path software will determine if theconsidered neighbor was the last U-turn neighbor in the list (222). Ifthe considered neighbor was not the last U-turn neighbor in the list,the process will select another U-turn neighbor and iterate until itfinds a viable U-turn neighbor or it reaches the end of the list of theU-turn neighbors. If the alternate path software reaches the end of thelist without finding a viable U-turn neighbor, it will conclude thatthere is no possible alternate loop free neighbor or U-turn neighborthat can serve as an alternate for the destination (224) and move on toconsider the next destination.

Although an example was described in which U-turn neighbors wereconsidered separately from other neighbors, the invention is not limitedin this manner as all neighbors may also be grouped together and rankedaccordingly. For example, in some instances the highest ranked U-turnneighbor may be more preferable than the lowest ranked regular neighbor.In this instance, ranking the regular neighbors and U-turn neighborstogether may enable all neighbors to be grouped together onto one listand ranked accordingly. The fact that a particular neighbor is a U-turnneighbor may affect the ranking of that neighbor in the list in thisembodiment.

Although an embodiment of the invention has been described in connectionwith an implementation in a routed IP network, the invention is notlimited in this manner as it may also be used in other networks where alink state routing protocol is being used. For example, an embodiment ofthe invention may be used in connection with a routing bridge (RBridge)network running a link state routing protocol such as IS-IS.Accordingly, the invention is not limited to implementation on an IPnetwork or in a router, but may also be implemented in other types ofnetwork elements such as switches or bridges.

The functions described above, including these described with respect toFIGS. 6 and 7, may be implemented as one or more sets of programinstructions that are stored in a computer readable memory within thenetwork element(s) and executed on one or more processors within thenetwork element(s). However, it will be apparent to a skilled artisanthat all logic described herein can be embodied using discretecomponents, integrated circuitry such as an Application SpecificIntegrated Circuit (ASIC), programmable logic used in conjunction with aprogrammable logic device such as a Field Programmable Gate Array (FPGA)or microprocessor, a state machine, or any other device including anycombination thereof. Programmable logic can be fixed temporarily orpermanently in a tangible medium such as a read-only memory chip, acomputer memory, a disk, or other storage medium. Programmable logic canalso be fixed in a computer data signal embodied in a carrier wave,allowing the programmable logic to be transmitted over an interface suchas a computer bus or communication network. All such embodiments areintended to fall within the scope of the present invention.

It should be understood that various changes and modifications of theembodiments shown in the drawings and described in the specification maybe made within the spirit and scope of the present invention.Accordingly, it is intended that all matter contained in the abovedescription and shown in the accompanying drawings be interpreted in anillustrative and not in a limiting sense. The invention is limited onlyas defined in the following claims and the equivalents thereto.

1-16. (canceled)
 17. A method of calculating a loop-free alternate paththrough a network from a source node to a destination node, the methodcomprising: selecting a neighbor node of the source node based onweighting information determined by at least one of network policy andnetwork traffic conditions; and calculating a loop free path through theselected neighbor node to the destination node.
 18. The method of claim17, wherein the weighting information comprises respective policyweights associated with the respective neighbor nodes to favor the useof some neighbor nodes over other neighbor nodes.
 19. The method ofclaim 18, wherein the respective policy weights are assigned torespective links connecting the source node to the respective neighbornodes to associate the policy weights with the respective neighbornodes.
 20. The method of claim 18, wherein the policy weights arestatically assigned.
 21. The method of claim 18, wherein the respectivepolicy weight for a neighbor node is adjusted when an alternate routepassing through the neighbor node is selected to provide an alternatepath.
 22. The method of claim 17, wherein the weighting informationcomprises respective traffic weights based on network trafficinformation associated with the neighbor nodes.
 23. The method of claim22, wherein the respective traffic weights are dynamically determined.24. The method of claim 22, wherein the respective traffic weights arebased on traffic being handled by neighbor nodes.
 25. The method ofclaim 22, wherein the respective traffic weights are based on residualcapacity of neighbor nodes.
 26. The method of claim 22, wherein thetraffic weights are based on statistical network usage informationassociated with the neighbor nodes.
 27. The method of claim 22, whereinthe respective traffic weight for a neighbor node is adjusted when analternate route passing through the neighbor node is selected to providean alternate path.
 28. The method of claim 17, wherein selecting aneighbor node of the source node based on weighting information,comprises: ranking the neighbor nodes based on the weightinginformation; and considering one neighbor node after another indecreasing order of ranking to determine whether a loop fee alternativepath from the source node to the destination node can be found.
 29. Themethod of claim 28, further comprising, upon finding a loop freealternate path: stopping consideration of neighbor nodes; and selectingthe found loop free alternate path for use.
 30. The method of claim 29,further comprising, upon not finding a loop free alternate path: rankingU-turn neighbor nodes based on the weighting information; andconsidering one U-turn neighbor node after another in decreasing orderof ranking to determine whether a viable U-turn path from the sourcenode to the destination node can be found.
 31. The method of claim 30,further comprising, upon finding a viable U-turn path from the sourcenode to the destination node: stopping consideration of neighbor nodes;and selecting the found viable U-turn alternate path for use.
 32. Themethod of claim 28, further comprising, upon finding one of a loop freealternate path and a viable U-turn path: stopping consideration ofneighbor nodes; and selecting the found one of a loop free alternatepath and a viable U-turn path for use.
 33. The method of claim 17,wherein the network is a packet switched network and the alternate pathsare for the routing of packets through the network.
 34. The method ofclaim 17, wherein the network is a link state protocol controllednetwork.
 35. The method of claim 17, wherein the method is initiatedresponsive to a link failure on the network.
 36. The method of claim 35,wherein the link failure is on a link between the source node and aneighbor node.